Running PM v5.0, build 16299 (579a307) under Windows 8.1 running on a SSD, Core i7-4930K CPU @ 3.4 GHz, 32 GB RAM, image files stored on another internal SSD:
When in the Contact Sheet window, the feature "Open in current Contact Sheet" works perfectly in most cases. However, on my system it fails when two large folders are involved.
Folder A contains 12,201 .CR2 raw files, plus corresponding .XMP files, totaling to 24,402 files. Folder B contains 10,831 .CR2 raw files, plus corresponding .XMP files, totaling to 21,662 files. I can open each folder by itself into a new Contact Sheet in approximately two seconds.
After opening a single Contact Sheet for either folder, Windows' Task Manager reports approx. 9% CPU utilization, and approx. 5% utilization of the SSD containing the image files, for a long time after opening the Contact Sheet -- but the Contact Sheet is visible to me, and responsive, after approx. 2 seconds. By "responsive," I mean that I can scroll down into the Contact Sheet quickly and smoothly, and as soon as I stop scrolling, the thumbnails for the images now visible in the Contact Sheet window are generated nearly instantly.
However, if I have Folder A open in a Contact Sheet, then attempt to open Folder B into that existing contact sheet, I get an hourglass icon which persists for minutes, until I force PM closed via the Windows Task Manager.
This behavior is the same if I have Folder B open as a Contact Sheet, then attempt to open Folder A into the existing Contact Sheet.